Another ESI 250 DE Universal Impedance Bridge

ESI 250 DE unit #2
ESI 250 DE unit #2 front panel

Everyone should have at least two of these ESI 250DE Impedance bridges.

Well that was not my first thought when pur­chas­ing this unit on Ebay.
I was still hav­ing some issues with the “Internal DC” detec­tor cir­cuit on my exist­ing unit, and the price of this unit was extreme­ly rea­son­able. After dozens of hours trou­bleshoot­ing the first unit with no solu­tion, my thought was to com­pare the two cir­cuits and sal­vage parts from the new unit if necessary.

Unit #2 arrived in what seemed to be good con­di­tion, with many years of dirt and dust buildup espe­cial­ly under the dials and knobs. As usu­al the first part of the project was to clean up the decades of grime and stick­er residue.

ESI 250 DE unit #2 after an ini­tial wipedown, before knob and dial removal
ESI 250 DE unit #2 after dial and knob removal

Part two of the cleanup was a thor­ough clean­ing of all switch­es and poten­tiome­ters with DeoxIt D5 con­tact clean­er, and lubri­ca­tion of mov­ing parts.
After the cleanup it was time to con­nect it up to my cur­rent lim­it­ed bench pow­er sup­ply and do some ini­tial test­ing.
Well, that was a dis­ap­point­ment. Nothing worked except for the bat­tery test position.

ESI 250 DE unit #2 main cir­cuit board with loose F3 mod­ule and bent ground strip

After inspect­ing the main cir­cuit board, I found the F3 mod­ule (black 13290) was very loose and wob­bly. All the sol­der con­nec­tions were in great shape with no signs of pre­vi­ous work, but I did notice that the top of the ground plane strip had an odd bend in it.
Upon de-sol­der­ing the F3 mod­ule I found that one of three leads had been sheared off leav­ing it dis­con­nect­ed from the board. After a quick repair I tried to reassem­ble the unit back in its case for test­ing with it’s 4 D‑cell bat­tery pack, but could­n’t get the threads to line up with the mount­ing hole. Upon inspect­ing the inter­nal mount­ing brack­et, it was obvi­ous that it had been severe­ly bent and twist­ed.
It began to look like the unit had sus­tained a sig­nif­i­cant impact to cause this much bend­ing, which also explained the sheared lead on the F3 mod­ule and dam­age to the ground plane strip.
The odd thing was that there was no exter­nal signs that this piece of test equip­ment had been dropped from such a height to cause this much damage.

ESI 250 DE unit #2 Internal dam­age to 1/8″ (3.2mm) alu­minum bracket
ESI 250 DE unit #2 Back side before clean­ing with no signs of impact

After straight­en­ing the brack­et and replac­ing a sheared screw the unit was test­ed again with the same results.
It was time to check every­thing for mechan­i­cal dam­age, but much to my sur­prise every­thing else oth­er than the main cir­cuit board was OK and all resis­tances were well with­in spec­i­fi­ca­tions.
It was time to de-sol­der the remain­ing two large items on the cir­cuit board.
The F2 “Detector Network” mod­ule was in good shape and its resis­tance and capac­i­tance mea­sure­ments were with­in spec­i­fi­ca­tions.
The F1 “Input Filter” mod­ule did have a sheared pin, but I could not test the mod­ule due to a lack of doc­u­men­ta­tion from the ESI man­u­al.
My curios­i­ty got the bet­ter of me so I decid­ed to open the mod­ule to see how it worked.

ESI 250 DE unit #2 Module F1 back side
ESI 250 DE unit #2 Inside the F1 module

After drilling out two dim­ples in the case which were mechan­i­cal­ly retain­ing the cir­cuit board, I was very pleased to find that the inter­nals were not potted.

ESI 250 DE unit #2 Module F1 with relay shield removed and Coto relay and reed switch

The F1 mod­ule cir­cuit con­sists of 4 capac­i­tors, 4 resis­tors, 2 diodes, and a shield­ed relay.

ESI 250 DE unit #2 Testing the relay for prop­er operation

After test­ing the relay and all the oth­er com­po­nents, I doc­u­ment­ed the inter­nals of all the F mod­ules for future reference.

ESI 250 DE F mod­ule schematics

Modules F2 and F3 are a Twin‑T fil­ter net­work and are used in the Detector and Generator cir­cuits respec­tive­ly.

Module F1 is a inter­est­ing cir­cuit with pin 1 being the detec­tor input which is con­nect­ed through R1 a 100K Ohm resis­tor in the first stage of a RC fil­ter to a diode lim­iter, C1 a 0.47 uF capac­i­tor and R2.
The next RC fil­ter stage is formed by R2 and C2, then R3 and C3 the next stage, then R4 and C4 the final stage before con­nect­ing to the reed relay con­tact and pin 4.
Pin 3 of mod­ule F1 is con­nect­ed to ground when the mode is set to either of the DC Detector modes, tying one side of the four capac­i­tors and the diode pair to ground.
When the ESI 250 DE is oper­at­ing in the “Internal DC” mode the relay cycles at a 22 to 25 Hz rate con­nect­ing pin 4 to pin 5 at that same rate.

ESI 250 DE F2 Module Bode plot for Twin‑T circuit
ESI 250 DE F3 Module Bode plot for Twin‑T circuit

Well, after jump­ing down that rab­bit hole of fil­ter design and reverse engi­neer­ing cir­cuits. I found that after repair­ing the bro­ken lead of the F1 mod­ule, the prob­lem was that the cir­cuit board card edge con­nec­tor was not mak­ing good con­tact with sock­et con­nec­tor fin­gers.
This was caus­ing a sev­er­al hun­dred Ohms resis­tance on one of the grounds. After a good clean­ing with DeoxIt D5 and some del­i­cate re-bend­ing of the con­tact fin­gers the unit was oper­at­ing cor­rect­ly.
This clean­ing and bend­ing on unit #1 also cor­rect­ed its “Internal DC” mode problem.

This unit also came with a mod­el 1325 recharge­able bat­tery cov­er that plugged direct­ly into the exist­ing bat­tery fix­ture con­tacts in the bat­tery com­part­ment. Unfortunately the Ni-cad bat­tery was not recov­er­able, so I am using stan­dard D‑cell bat­ter­ies with­out the charg­ing circuit.

ESI 250 DE Units 1 and 2 side by side

I now have two func­tion­al ESI 250DE Impedance Bridges. My favorite is still the Grey themed unit with it’s non-stan­dard round meter. The grey themed unit has all plas­tic parts of the knobs, dials, and jacks in a light grey col­or with a medi­um grey face­plate, while the black themed unit has all it’s plas­tic pieces in black. The grey themed unit has it’s out­er case paint­ed with a grey-blue col­or with the oth­er unit in a dark­er blue-grey col­or, and both units hav­ing a black plas­tic cov­er. My guess would be that the col­or schemes have to do with the age of when the units were pro­duced. The most recent dat­ed com­po­nents in unit #2 are from ear­ly 1988, and unit #1 was built in 1971.
More inter­nal pic­tures of unit #1 in the March 2018 blog post.

While most peo­ple do not need two of these ESI 250DE Impedance Bridges, I do rec­om­mend hav­ing at least one of these pieces of test equip­ment on your elec­tron­ics bench for test­ing com­po­nents and have found the 250DE to be extreme­ly accurate.

One Reply to “Another ESI 250 DE Universal Impedance Bridge”

  1. Hi Greg,

    as own­er of an ex German Army ESI 250DE I appre­ci­ate the infor­ma­tion on your home­page about this fine piece of mea­sure­ment equip­ment. I like to have the ser­vice doc­u­men­ta­tion of my equip­ment at hand, so the man­u­als you scanned are very help­ful. By the way, the US army also used this instru­ment, so there is a test and cal­i­bra­tion man­u­al available:

    TB 9–6625-1014–24

    You can find it here:

    Then select Search Type: Pub Number
    Then enter the TB num­ber above into the Search cri­te­ria field and click “Search”.
    The man­u­al is avail­able as PDF.

    Hope this is help­ful for oth­er ESI 250DE owners.


    Rainer DG1SMD

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